The present invention relates generally to methods and apparatus for continuous, non-invasive monitoring of the hemodynamic state and the arterial blood pressure of a subject.
A method for the determination of non-invasive beat-by-beat (continuous) systolic and diastolic arterial blood pressures has long been desired in patient monitoring. Automatic blood pressure cuffs can be used for this application, by inflating them as rapidly as is possible; however, this provides blood pressure data only at 1 to 2 minute intervals, and each inflation can be painful, especially for elderly or hypertensive patients.
A common technique for continuously measuring blood pressure is to insert a saline-filled catheter through the patient's vascular system to the point at which it is desired to perform the measurements. The catheter is connected to a pressure sensor, which measures the pressure in the vessel. An alternative method uses a catheter with a pressure sensor at the tip that directly senses the blood pressure. However, these techniques involve making an incision through the patient's skin and inserting the catheter into a blood vessel. As a consequence, this invasive procedure entails some risk of complications for the patient.
An indirect, non-invasive process for continuously measuring blood pressure is based on the pulse transit time (PTT) which is the difference in time required for a pressure pulse generated from a heart beat to propagate between two points in the vascular system. One apparatus for this technique includes an electrocardiograph that senses electrical signals in the heart to provide an indication when a blood pulse enters the aorta. A pulse oximeter is placed on an index finger of the patient to detect when the blood pressure pulse reaches that location. The pulse transit time between the heart and the index finger is measured and calibrated to the existing blood pressure that is measured by another means, such as by the automated oscillometric method. Thereafter changes in the pulse transit time correspond to changes in the blood pressure. Generally, the faster the transit time the higher the blood pressure. Thus, changes in the pulse transit time can be equated to changes in the blood pressure. However, the electrocardiograph (ECG) senses electrical signals in the heart, which do not indicate the point in time when the blood pressure pulse actually leaves the heart upon the mechanical opening of the aortic valve. A time interval of varying length, known as the cardiac pre-ejection period (PEP), exists between peaks of the QRS wave of the electrocardiogram signal and the aortic valve opening. The inability of prior pulse transit time-based monitors to account for the cardiac pre-injection period results in an inaccurate measurement of the pulse transit time and thus blood pressure. Additionally, changes in the compliance of the blood vessels also affect the pulse transit time estimates. Chronic changes in arterial compliance occur due to aging, arteriosclerosis or hypertension. Arterial compliance can also change acutely due to neural, humoral, myogenic or other influences. Previous monitoring systems have been unable to separate changes due to compliance from changes due to blood pressure. As a consequence, some degree of inaccuracy has existed in calculating blood pressure from the variation of the pulse transit time.
Other indirect, non-invasive techniques for continuously measuring blood pressure are based on biomechanical models to convert continuous indirect arterial measurements into blood pressure readings. These continuous measurement techniques rely on a static biomechanical model derived from either empirical data or a calibration of the model using an independent pressure reading. Similar to PTT, these techniques lack the ability to adapt to acute physiological changes affecting the biomechanical model, such as changes in the arterial compliance.
Two common continuous measures used in these techniques include tonometry and plythesmography. Tonometry measures variations in the pressure at the surface of the skin induced by changes in the arterial pressure. The tonometric sensors are typically placed over small arteries on the hand or wrist, and the measurements are sensitive to motion and alignment. Plythesmography measures changes in the volume of a limb, such as the arm. Since plythesmogrpaphy measures the change in the total volume, contributions from the arterial and venous systems cannot be separated contributing to errors in the arterial blood pressure measurement.
Therefore, an improved blood monitoring system is desirable to address one or more of the aforementioned issues.